Well by Gen 7 they kind of need to balance Fairy; even after 3 years of Gen 6, Fairy is still up there as one of the most consistently powerful type.
Despite the existence of Fairies, the Steel-type actually became weaker in the transition from Gen 5 to Gen 6 simply because Sucker Punch and Shadow Ball aren't resisted anymore.
People are actually resorting more to Fire-types than Steel-types to mitigate the Fairy infestation (at least in VGC Doubles). This then made Fire a really good type (it was already really good since Gen 2 offensively, only offset by how defensively weak it was when Ground-types started to rule around 4th gen), resulting in a lot of Fire-types being very viable, alongside the buffs to Will-O-Wisp and Flare Blitz getting more distribution now.
Meanwhile, Poison is still pretty weak, mostly because the Pokemon that do have the type are weak in their base stats instead. Most viable Poison-Pokemon are known more for their other typing and their ability to not die in one Earthquake, most notably Gengar and Venusaur.
Back to Fairy, in the context of VGC, back in 2014 we had a number of viable Fairy-types; Mega Mawile was part of the big 3, Azumarill could choke a lot of options in certain situations, Gardevoir and Florges were easily specialized to spread basic damage with Dazzling Gleam. In 2015, Sylveon and Gardevoir gained Hyper Voice, Togekiss became legal, Clefairy and Clefable became the best Follow Me users, Mega Mawile is still good... this is all despite Heatran being legal again. Now in 2016, we have the Xerneas win-button, forcing people to build teams trying to counter it, and typing won't work because it's usually paired up with P. Groudon, so it had to be roundabout ways... which a clever use of their best friend Mega Kangaskhan (who, by the way, centralized both 2014 and 2015) could easily mitigate. And when you look at the second most popular trio, the Rayquaza-Kyogre duo is usually backed up by another Fairy... Mega Mawile. Togekiss and Clefairy are still being used as Follow Me users too.